Side airbag apparatuses have been proposed in which an airbag is accommodated in a folded state at a roof side rail portion at a B pillar portion, behind a roof head lining. During deployment, the airbag can release into the vehicle compartment between the roof head lining and a B pillar garnish. While it can be important to provide such an airbag release location between the roof head lining and B pillar garnish, it can be undesirable for the B pillar garnish to sag apart from the roof head lining. Such sagging can result in a visual gap between the head lining and B pillar garnish. What is desired is a retention structure that can hold the B pillar garnish in place, even during an airbag deployment.